The research program focussed mainly on isolating, purifying, and characterizing the snail-emitted "miraxone" that alters behavior of Schistosoma mansioni miracidia, and probably enhances their snail-finding capacity. Water "conditioned" by Biomphalaria glabrata was collected and analyzed by various methods, all rigorously controlled; these analyses disclosed that the miracidial stimulant in "conditioned" water was magnesium ion. This finding casts new light on the phenomena whereby miracidia aggregate and swim excitedly within a chemically "active space." The finding also raises certain fundamental questions, e.g., does the magnesium affect ciliary or muscular motility in the rapidly turning, stimulated miracidium? We have also developed a new bioassay to measure miracidial chemosensitivity. The system is simple, rapid, and reproducible, and it quantifies miracidial aggregations that do (or do not) occur in response to test substances. The apparatus is a flat-bottomed circular well notched to receive a snug-fitting dam that bisects the well. (The well's shape suggests the Greek letter phi, hence "phi-chamber.") Miracidia swim freely in the "open" well, test material is introduced at one edge, and 30 sec. later the dam is inserted and iodine added to kill and stain the miracidia for counting. Many tests can be run expeditiously, and the procedure lends itself to developing dilution-response curves for miracidial stimulants or inhibitors. Thus, while Co2 ion, Be 2 ion, and Mn 2 ion stimulate miracidia, none of them is as potent as Mg 2 ion. Various biogenic amines and their precursors have also been screened, but few stimulated miracidia at nominally physiologic concentrations. BIBLIOGRAPHIC REFERENCE: Stibbs, H.H., Chernin, E., Karnovsky, M.L., and Ward, S. 1976. Magnesium emitted by snails alters swimming behavior of Schistosoma mansoni miracidia. Nature 260:702-703.